The inhomogeneous optical layer is a layer which is so formed that the optical refractive index varies from its inner surface contacting with the substrate to its outer surface or exposed surface, and a special inhomogenous optical layer in which the refractive index varies monotonously is used widely as an optical film for preventing reflection of light or as an optical film for absorbing light selectively depending on the wave length of the light.
In a known method for forming such an inhomogeneous optical layer, the substrate is fixed in a vacuum coater vessel including two evaporation sources so that a surface of this substrate faces one evaporation source for high refractive substance as well as the other evaporation source for low refractive substance, and both the sources are energized in order that both the substances are evaporated in the sources, emitted from them and deposited on said one surface. Each of the deposition rate of each of the substances is watched by an appropriate monitor, and the input energy of each of the sources is controlled in order to vary the deposition rate of each of the substances with time according to the program. For example: if the depositing rate of the high refractive substance is decreased monotonously with time and that of the low refractive substance is increased monotonously with time; in the layer made of both the substances deposited on said one surface, the content of the high refractive substance decreases monotonously in the direction of the outer surface of the layer and the content of the low refractive substance increases monotonously in the same direction, and therefore the resultant refractive index in the layer decreases in said direction.
However, the known method has disadvantage that it is impossible to form an inhomogeneous optical layer on a surface of a long, wide or large substance because the substrate has to be fixed in the vacuum vessel, and the operation is troublesome because it is necessary to vary the deposition rate with time.